Lapping machine



Nov. 13, 1962 Filed May 5, 1959 A. MEYERHOFF ETAL LAPPING MACHINE 7 Sheets-Sheet l Alfred Meyerhoff Bruce L.S'fubbs ATTORNEY Nov. 13, 1962 A. MEYERHoFF ETAL 3,063,206

v LAPPING MACHINE Fil-ed May 5, 1959 y '7 Sheets-Sheet 2 Figa.

r452 A l N0V- 13, 1962 A. MEYERHOFF ETAL 3,063,206

LAPPING MACHINE Filed May 5, 1959 '7 Sheets-Sheet 5 Fig.3. l

Nov. 13, 1962 A MEYERHOFF E'TAL 3,063,206

LAPPING MACHINE Filed May 5, 1959 7 Sheets-Sheet 4 e5 Fig.4.

NOV 13, 1962 A. MEYERHOFF ETAL 3,053,206

LAPPING MACHINE Filed May 5, 1959 7 Sheets-Sheet 5 NOV- 13, `196.2 A. MEYERHOFF ETAL 3,063,206

LAPPING MACHINE '7 Sheets-Sheet 6 Filed May 5, 1959 United States Patent 3,063,206 LAPPING MACHINE Alfred Meyerhotf, Greensburg, and Bruce L. Stubbs, Westmoreland City, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed May 5, 1959, Ser. No. 811,118 6 Claims. (Cl. 51-161) The present invention relates to lapping machines, and more particularly to methods and means for precisely controlling the lapping of semiconductor materials.

Among the most sensitive operations of semiconductor manufacture are the removal of surface dam-age found on wafers of the crystal material after the wafers have been sliced from a grown single-crystal bar and the precision and accuracy to which parallelism, flatness and thickness can be held. Removal of the surface damage and reducing the wafer to a suitable thickness for etching is performed by a lapping machine. Conventional practice is to lap the crystal Wafer to a size somewhat larger than the desired size and thereafter reduce the wafer to proper size by exposure to an etching solution. Prolonged etching, however, is not satisfactory due to the irregularity with which the etching sol-ution attacks the wafer and the reduction of the wetting ability of the wafer, that is the ability of the wafer to adhere to a wafer of opposite polarity when `forming a junction. Our invention provides methods and means for accurately nishing the wafers to proper size during the lapping operation. By exacting control of the lapping operation, the wafer can -be reduced to proper size and merely flashed in the etching solution thereby overeen ing the problems pertaining to the etching process.

The reduction of semiconductor material wafers to such small thicknesses creates many problems. The wafers have an increasing tendency to buckle in the machine as their thickness is reduced. Accurate gauging of the thickness of the wafers is necessary to determine the progress of the lapping operation. The type of abrasive slurry as well as the particle size can vary the rate at which the thickness of the wafer is reduced. When lapping wafers to such small thicknesses, clogging of the abrasive slurry or erratic ow of the slurry can easily result in destruction of the semi-conductor material wafers.

An object of our invention is to provide means for reducing the stresses imposed upon the wafers during Voperation of the lapping machine thereby reducing breakage.

Another object of our invention is to automatically gauge the thickness of the semiconductor material wafers while the lapping machine is in operation.

Another object of `our invention is to provide a system for automatically controlling the flow of abrasive slurry.

Another object of our invention is to provide for removing the finished wafers =frorn the lapping machine after the proper lapping has been accomplished.

Further objects and advantages of our invention will be seen from the following detailed description taken in conjunction with the drawings, in which:

FIGURE l is a side elevation view, partly in section, of a lapping machine embodying our invention;

FIG. 2 is a lfragmentary side elevation view of FIG. l, enlarged in size, showing our invention;

FIG. 3 is a plan view of FIG. 2;

FIG. 4 is an enlarged detail View, partly in section, of a portion of FIG. 2;

FIG. 5 is a further enlargement, partly in section, oa portion of FIG. 4;

FIG. 6 is a view, partly in section, taken along the line VI-VI in FIG. 2;

ICC

FIG. 7 is a schematic diagram of the pneumatic system employed in our invention;

FIG. 8 is a schematic diagram of the electric circuitry employed by the invention;

FIG. 9 is a side elevation view of a detail of our invention;

FIG. l0 is a front elevation view of FlG. 9 and FIG. l1 is a schematic diagram of the negative liuid pressure system employed in our invention.

It is well known that lapping machines basically consist of two lap plates between which carriers or workholders driven by external means, slide the wafers in an abrasive slurry. The resultant rubbing or grinding the inner surfaces of the plates. The slurry charges the plates with abrasive particles thereby assisting in theremoval of the surface of the wafers.

FlG. s1 illustrates the general layout of a lapping machine embodying our invention wherein a conventional eddy current adjustable speed drive unit 2 is located in the lower portion of the machine. A work table 4 and control cabinet 6, as well as the area of the machine where the lapping is accomplished, indicated generally by the reference 8i, are located above the drive unit 2. The drive unit 2 is secured to a base plate' 1'0 and basically consists of a squirrel cage motor 12 connected to a right'angle gear box 14 through a coupling 16. A belting arrangement indicated at 18 connects the gear box 14 to a shaft 20 -andsuitable transmission gearing 21 extending into the lapping area 8. Within the housing of the squirrel cage mjotor 12 there is containedy an eddy currentl couplingV (not shown) which, by varying its field strength can be made to slip in accordance with the desired torque and speed on the shaft Ztl'.

Referring to FIG. 6,A the shaft 20 enters the area 8 where the l-apping is accomplished and through the suit-v able transmission gearing 21 is so connected to a sun gear 22 and ring gear 24 that the two gears run atV different speeds. Work carriers 28, in the form of plane` tary gears located between the sun gear 22 and ring gear 24, provide pockets 30 wherein individual semiconductor material wafers are placed as indicated at 32. The drive shaft 20 drives the sun gear 22 and ring gear 24 at relative speeds determined by the adjustable speedl drive unit 2. The planetary gearsor Work carriers 23 rotate around the sun gear 22, and provide a force on the wafers 32 causing them to slide between the inner sur'- vfarces of a top lap plate 25 and a bottom lap plate 26..

The weight of the top plate 255 rests, upon the wafers'32 and provi-des a force which urges the wafer surfaces against the inner surfaces of the top and bottom lap' The bottom -lap plate 26 is secured stationary' plates. to the machine. The topi lap plate 25 is not permitted to rot-ate relative to the planetary work carriers 28 by a strong-back 34 hinged on the machine and placed over the top lap plate 25 to engage stud pins 36 protruding from the top plate. The top -la-p plate 25 is free, however, to move in the vertical direction and as the wafer thickness is reduced, the top plate will descend closer to the bottom lap plate 26. An abrasive slurry is introduced between the top `and bottom plates to increase the grindn chine on numerous occasions to determine the extent of Ithe material removed from the wafer during the preceding time interval of operation. Since lapping time is a function of volume to be removed and the diameter of the wafers varies greatly, no precise time dilerential or interval can be established to arrive at the desired thickness. In addition, the time interval method h-as another variable. 'Ihe type of abrasive material used, as well as the particular size of the abrasive, can vary the necessary ti'me interval of operation considerably.

FIG. 2 shows means whereby our invention allows the automatic gauging of the thickness of the wafers 32 as the lapping operation is taking place. An air probe 52 is secured to the rigidly held bottom lap plate 26. The air probe 52 .has a contact point 54 which is positioned to be actuated by a downward extending micrometer adjustment screw 56 rigidly secured to the upper lap plate 25. 4It is to be recalled that the upper lap plate 25 has unrestricted motion in the vertical direction in that it merely rests upon the wafers 32. In fact, the Weight of the top lap plate bears a direct relation to the amount of material that will be removed from the wafers 32 as the work carriers 28 push the wafers between the bottom and top lap plates. As the thickness of the work piece is decreased the upper lap plate 25 and therefore the micrometer adjustment screw 56 will descend unto the contact point 54 of the air probe 52. The -air probe 52 bleeds pressurized air in proportion to the thickness of the wafers 26. An air gauge mounted in the control cabinet 6 reads the pressure loss within the air pressure system hereinafter described and is calibrated to show the progress of the lapping machine. The air gauge is equipped with a pressure sensitive rel-ay, which initiates an electrical circuit automatically stopping the machine when the desired thickness size of the wafers is reached.

Although mechanical gauges could be used to measure the thickness of the wafers 32 they have proven unsatisfactory Ibecause of their inability to dampen out the mechanical machine vibrations inherent in the moving parts of the lapping machine. As a matter of fact, quivering of the indicator needle over a range of several thousandths of one inch makes accurate readings nearly impossible. It is obvious that the air probe gauge provides a rather simple and a quick means for automatically measuring the thickness of the wafers 32 while the lapping is in progress. Since air is a compressible fluid it also provides a dampening effect upon mechanical machine vibrations. l

As the wafer surfaces are ground away the compressive stresses resulting at the point of contact between work carrier 28 and wafer 32 become more severe. These stresses increase exponentially as the thickness of the wafers 32 decreases. From FIG. 6 it is apparent that the wafers 32 are moved about between the lap plates by means of the work carriers 28. Each wafer 32 is of smaller size than the pocket 30 in which it is contained. The side walls of pocket 30.engage the edge of the wafer 32 and urge it to slide between the plates 25 and 26. As the wafer 32 becomes thinner the area over which the force of friction can be applied by the work carrier 28 becomes smaller because of the reduced thickness of the wafer. The force of friction is the force needed to make the wafer 32 slide between the lapping plates. Depending upon the contiguration of the wafer, it is possible that the force of friction be entirely applied to a point contact on the particular wafer. Because of the extreme thinness of the wafers 32 the friction force can 4cause the compressive stresses Ibuilt up within the wafers 32 to become excessive and cause the wafer to crumble, which, in turn, can result in the ruination of not only that particular wafer 32 but all of the wafers within the machine as well. The yfragments of the shattered 'wafer 32 can break the other wafers in the machine.

An examination of the lforce of friction required to slide the wafers between the top and bottom'lap plates will idicate that its magnitude is equivalent to the cof a `false thickness reading could result.

eliicient of friction multiplied by the force pushing the wafer surface and the lapping plate surfaces together. The magnitude of the force pushing the surfaces together is the weight of the top lapping plate 25 as it rests upon the w-afers 32. The coefficient of friction, which is the key to rubbing or grinding away the wafer material within the lapping machine, can only be partly controlled by Varying the type of abrasive slurry between the lap plates. However, the weight bearing upon the wafers 32 can lbe readily controlled. In Iaccordance with our invention, means have been provided for reducing the weight .bearing upon the wafers 32 as they approach the thickness wherein they can be subjected to destroying stresses.

The measurement and the control of the force pushing the surfaces together is provided by a lifting bar 60 which is attached to the upper lap plate 25 by means of hooks 62 secured to the exposed top surface of the top lap plate 25. The lifting bar 60 in turn is controlled by means of a cable 66 which is adapted to pull upon the hub portion 68 of the lifting bar 60. The cable 66 is connected to one end of a spring scale 70. A turn-buckle Vernier control 72 on the opposite end of the spring scale 70 can be rotated to pull the cable 66 thereby reducing the weight of the top lap plate 25 upon the wafers 32. This arrangement allows very close control of the weight bearing upon the wafers 32 which will hereinafter be referred to as the force pushing the surfaces together. Means have been provided for visually determining the force pushing the surfaces together by the calibration of the spring scale 70. During initial opera-tion, the turn-buckle or screw-nut system 712 may be completely relieved allowing the full weight of the top lap plate 25 to rest on the wafers 32 as they slide between the lapping plates. As the wafers 32 approach the desired thickness, as determined by the micrometer adjustment screw 56, the weight of the top lap plate is reduced by withdrawing the turn-buckle Vernier control thereby reducing7 the force pushing the surfaces together. In turn, the necessary force of friction to slide the wafer 32 between the lap plates can also be reduced. In such a manner the stresses to which the wafers 32 are subjected during the lapping process are controlled and wafer breakage is minimized. Our invention allows all the adjustments to be made while the lapping machine is in operation.

It is to be noted that any means for determining the thickness of the wafers by measuring the separation between the top lap plate 25 and the bottom lap plate 26 includes not only the wafer 32 but also the grinding particles of abrasive slurry underneath and on top of the Wafer surfaces. Should the introduction of the abrasive slurry between the lap plates be uneven and sporadic Uneven lapping of the wafers could also occur which is highly undesirable. With conventional machines the abrasive slurry is quite often added manually by the operator of the machine. This arrangement has the obvious disadvantages just discussed.

Our invention provides for uniform automatic distribution of abrasive slurry between the top and bottom lap plates. That distribution is provided by an abrasive mixing bowl 42 secured to a mounting post 44 by means of an extension arm indicated at 46. The abrasive and a suitable liquid is continually mixed within' the bowl 42 by means of an agitator 43 driven by suitable means such as an electrical motor 48 shown mounted above the abrasive mixing bowl 42. As illustrated in FIG. 4 the abrasive slurry is bled from the mixing bowl and fed to entrance ports or wells 40 located in the top lap plate 25. The wells 4G lead to the inner surfaces of the top and bottom lapping plates.

The abrasive mixing bowl 42 has within its side wall 88 a plurality of oriiices 80. Four spaced orifices 80 have been shown, each extending through a cover plate 81 and the side wall 88. The abrasive slurry leaves the mixing bowl 42 through each orifice 80 and enters a small spout 83. A feed wire `50 secured at one end to the spout 83 by means of a screw clamp 3S allows the abrasive slurry to drip into the wells 40. Flow of slurry from the mixing bowl is continuous. However, the abrasive slurry has a tendency to clog the orifices 80 thereby stopping flow of the slurry to the lapping area. Our invention overcomes the clogging .of these orifices by piercing each orifice 80 with a probe 8,2 actuated by an air cylinder 84 in a timed sequence. The time interval for piercing the orifices 80 is regulated by an external timer, described hereinafter.

From FIG. 5, it can be seen that at the time when the orifices 80 are to be pierced, the face 86 of each air cylinder 84 is moved adjacent the cover plate 81 as the probe 82 enters the orilice. A recess 90 in the face 86 is provided with an O-ring 92 which seals the area between the face 86 and the side wall 88 thereby preventing leakage of abrasive slurry during the piercing operation.

While four orifices 80 and four feed wires 50 have been supplied, it is obv-ions that any number could be used. As illustrated, each wire 50 deposits slurry into an entrance ywell 40 within each quadrant of the lapping plates. It is readily apparent that a greater number of orifices 80 and feed wires S0 would result in even more uniform liow of abrasive slurry to the inner surfaces of the lapping plates.

It will be of considerable assistance in understanding the means by which we practice our invention by considering the operation of the control circuits used with the lapping machine. Like reference characters have been used to designate like or corresponding parts discussed previously. A schematic diagram of the pneumatic system is shown in FIG. 7. A globe valve 100 provides means for controll-ing the flow of compressed air from a supply indicated at 102. T-he compressed air is regulated and gauged by the pressure regulator 104 and gauge 106. The compressed air is then fed through a four-way elbow i107 to the automatic thickness gauge circuit.111 and the slurry flow control circuit 1.13. In the thickness gauge circuit 1111, measuring the relative distance -between the topy lap plate aud the bottom lap plate 26, the air probe 52 will, upon the wafers 32 reaching the desired thickness, open a self-contained orifice thereby -bleeding .the air pressure in the circuit 1111. The gauge 10S measures the drop in back pressure, indicates the distance of separation and is equipped with a sensitive relay 110` which initiates an electrical circuit stopping the lapping machine when the desired thickness has been reached. A filter 112 assists in maintaining the accuracy of the air probe 52.

The slurry control circuit 1i13 actuates the air cylinders 84, each of which operates a probe -82 in the manner discussed previously. A three-way selector valve 118 controls the action of the air cylinders 84. The three-way selector valve 118 is capable of either by-passing or connecting a solenoid valve 1-16 which is controlled by electrical means indicated by the coil i114. The air cylinders 84 are spring loaded so that the probes 82 are retracted from the orifices 80 in the absence of actuating air pressure. When the lapping machineV is in operation, the three-way feed valve 118 will be rotated to connect the solenoid valve `116 into the circuit `113 so that upon a proper time sequence, as determined 4by an electrical timer, the solenoid valve y1116 will open forcing each spring biased air cylinder 84 to move its probe 82 through the orifice 80. A flow control valve 120 provides means for controlling the distance that the probe 82 will advance within the oriice 80:.

An electrical schematic diagram is shown in FIG. 8. Power lines 200, and a neutral line 202, provide electrical power to operate the lapping machine upon closing of the load circuit breaker 204. The eddy current adjustable speed drive unit 2 is laid out generally at the top of FIG. 8 as indicated at 206. An amplifier circuit 208, located in the lower half of FIG. 8 furnishes means for receiving a signal from the air probe 52 and stopping operation of the lapping machine when the desired thickness is reached. The motor 48 which drives the agitator within the abrasive slurry mixing bowl 42 is connected across a power line 200 and the neutral line 202 through a line switch 210. A vacuum pump motor 2l2 is also connected in the same manner through another line switch 214. The vacuum system to which the vacuum pump motor 212 is connected will be discussed hereinafter.

As previously mentioned, the adjustable speed drive unit 2 includes a squirrel cage motor y12 connected to an eddy current coupling. The drive motor 12 is connected across the power lines 200 and allowed to come up to synchronous speed by means of contacts 214- closed by a closing coil 216 which is energized when the line switch 210 is closed. A line switch 218 connects the remainder of the electrical control and the primary side of a control transformer 220 within the amplifier circuit 208 across a power line 200 and the line neutral 202.

The seconda-ry side of the transformer 220 is centertapped to ground 222. A full-wave rectifier tube 224 is connected across the secondary of the transformer with the direct current output therefrom being filtered by the capacitor 226. A closing coil 228 is in parallel with the capacitor 226 and in series connection with a control tube 230. The control tube 230 is used in a switching mode and is non-conductive when negative grid voltage is impressed upon its control grid 232. Negative grid voltage is supplied to the grid 232 by tapping the transformer 220 through a biasing rectifier 242 and a dropping resistor 244 with an RC filter circuit 246. With negative grid voltage the tube 230 does not conduct to ground 222. Therefore, the closing coil 228 remains deenergized.

When the semiconductor material wafers 32 are positioned in the machine for lapping, the air probe S2 determines that the wafers are oversize. At this time the sensitive relay 110, connected to operate with the air probe S2, is closed thereby grounding the negative grid voltage. Provisions for manually grounding the negative grid voltage if desired is provided by the line switch 115.

The grid 232 becomes more positive causing the tube 230 to conduct and simulate a switch in the closed position. The closing coil 228 is energized andV closes contacts 248 and 250 in the push button and oversize indicator light circuit respectively. A third contact 296 opens the audio alarm circuit.

The lapping machine is now ready for the operator to commence the lapping operation. A potentiometer 252, which controls excitation to the eddy current clutch and hence the speed of the driving shaft 20- is placed at its zero speed position. A microswitch 254 in the push button circuit is closed by a mechanical interlocking arrangement with the potentiometer 252. The mechanical interlocking will be described hereinafter.

The push button circuit has a momentary contact start7 push button 256 and stop button 258. Upon closing of the start button 256 the relays 260` and 262 are initiated by their control coils 264 and 268 respectively. The normally open contacts 270 and 272 close to by-pass the start pushbutton 256 and connect the eddy current clutch coil 274 into the excitation circuit 206 respectively. The closing of the normally open contact 270 allows a timer 27S to be energized. The timer 278 in turn drives a cam 280 that closes the contacts 282 in a timed sequence which determines when the operating coil -114 of the air solenoid valve `116 is to be energized. The air solenoid valve .1,16 causes the probes 82 to pierce the orifice in the mixing bowl 42 as described previously. AIt is to be noted that the air valve 116 is energized immediately upon closing of the switch 218 so that the probes 82 are inserted within the orices 80 until a normally closed contact 284 is opened upon energization of the control coil 264. The contact 284 effectively removes the jumper across the cam contacts 282 and allows the 7 timing sequence to begin. A normally open contact 276 is closed by the same control coil 264 thereby by-passing the microswitch 254.

Referring to the eddy current drive unit shown generally at 206, a thyratron tube 286 is connected to be effectively in series with the clutch coil 274. The conductivity of the thyratron 286 is controlled to properly energize the clutch coil 274 for the desired lapping speed. A governor generator 288, connected to rotate with the output side of the eddy current clutch provides a tachoineter feedback voltage which is rectified and compared with a control voltage or bucking voltage resulting from the rectiiiers 250 and 292 connected to the terminals of the potentiometer 252. The voltage difference is modulated by grid control of the firing of the thyratron tube 286 thereby supplying the desired excitation to the clutch coil 274 for the desired lapping speed.

It will be recalled that as the thickness of the wafers 32 is reduced the stresses to which the wafers are subjected increase tremendously. Therefore, during any startup of the lapping machine it is desirable to bring the drive shaft 20 up to speed slowly so that the initial break-away torque will not cause the work carriers 28 to shimmy with the result that the wafers could shatter. The normally open Contact 272 will not allow the eddy clutch coil 274 to be energized until the potentiometer 252 is placed at zero speed position closing the microswitch 254. The mechanical interlocking of the microswitch 254 with the potentiometer 252 can be more readily seen from FIGS. 9 and l0. When the operating handle 290 of the potentiometer 252 is in the zero speed position, the spring biased operating lever arm 292 of the microswitch 254 will be fully extended thereby maintaining the switch in the closed position. As the handle 290 is rotated to increase the speed, the operating lever 292 will force the microswitch 254 to open but not until the control relays 260 and 262 have been energized thereby bypassing the microswitch 254.

During lapping the air gauge 108 (FIG. 7) can be read to determine the separation of the top and bottom lap plates 25 and 26. In this manner a visual means to determine the progress of the lapping is supplied for the machine operator. When the thickness of the wafers approaches a point where the stresses on the wafers could destroy them, the force pushing the surfaces together can be reduced by means of the turnbuckle Vernier control 72, and the friction force acting upon each wafer can likewise be reduced by adjustment of the potentiometer 252. As the lapping continues and the wafers reach the proper thickness, the air probe 52 signals its relay 110 which opens. The relay 110 no longer grounds the negative grid voltage from the rectifier 242 and dropping resistor 244, and the control tube 230 ceases to conduct. The tube 230 in turn deenergizes the closing coil 228 thereby causing the contacts 248 and 250 to open and assume once again the position shown in FIG. 7. A contact 296 also controlled by the coil 228 closes, energizing a green indicating light 298` in parallel with a buzzer 300 providing a visual and audio signal indicating that the wafers 32 have arrived at the proper thickness. At the same time the red light indicator 302 is deenergized indicating that the machine has shut down. The pushbutton circuit is opened deenergizing the ope-rating coils 264 and 268. Upon deenergization of these two coils their contacts once again assume the position shown in FIG. 8. Therefore, the contact 272 is once again open removing excitation to the clutch coil 274 of the eddy current disc drive. The timer 278 is again bypassed by the control contactor 284. When this happens the air control solenoid 114 is again connected across the line and energized, which initiates the probes 82 into the orifices 80. When desired the operator may turn the three-way feed valve 118 to bypass the solenoid valve 116.

The one last operation is accomplished by our invention. Since the lapping machine is capable of reducing the thickness of the wafers to extremely small proportions, upon removal of the top lap plate 25 it Will be found that the wafers 32 adhere by suction to the bottom lap plate 26. Because of the shape of the wafer it is extremely difiicult to pick up the wafers from the bottom lap plate 26. Therefore, a vacuum system has been incorporated in our invention whereby from FIG. l1, a vacuum pump 400 connected through an Erlenmeyer flask 402 and a filter 404 to a pick up bellows 406 provides suction to grasp the wafer 32 and evenly distribute the pickup force over the entire surface of the wafer 32. The filter 404 and Erlenmeyer fiask 402 have been found necessary to withdraw the abrasive slurry dust which is foreign material to the vacuum pump 400. The vacuum pump is driven by the electric motor 212 which is connected across a power lead 200 and the neutral lead 202 upon closing of the switch 214. When the pickup bellows 406 is not in operation it is mounted upon the mounting post 44 located on the lapping machine as can be seen from FIG. 2.

Our invention provides means for automatically gaug ing the thickness of the semiconductor material wafers as well as controlling the stresses acting upon the wafers while the lapping machine is in operation. At the same time automatic means has been provided for evenly distributing the abrasive slurry within the lapping machine and preventing clogging of the orifices within the mixing bowl 42. The finished wafers of extremely thin thickness are then removed from the bottom lap plate 26 by means of the vacuum pick up device 406.

While our invention has been described with a certain degree of particularity for the purpose of illustration, it is to be understood that all equivalents, modifications, and alterations within the spirit and scope of our invention are herein mentioned to be included.

We claim as our invention:

l. In a lapping macine for reducing the thickness of wafers comprising in combination a top lap plate and a bottom lap plate separated by the thickness of the wafers to be lapped, the top lap plate resting upon the wafers and exerting a force thereon equivalent to its weight, variable means for providing a friction force upon said wafers for causing them to slide across the inner surfaces of said plates and for causing said friction force to decrease as said plates come closer together, and means vfor reducing the effect of said first mentioned force while lapping of the wafers is in progress.

2. In a lapping machine for reducing the thickness of wafers comprising in combination, a top lap plate and a bottom lap plate separated by the thickness of the wafers to be lapped, the top lap plate resting upon the wafers and exerting a force thereon equivalent to its weight, a plurality of openings spaced about the outer periphery of said top lap plate and extending therethrough, an abrasive slurry mixing bowl disposed above said top lap plate, a plurality of orifices through said mixing bowl bleeding abrasive slurry to said openings in the top plate, a pneumatically operated probe aligned with each orifice, and means for causing said probes to periodically pierce said orifices.

3. In a lapping machine for reducing the thickness of wafers comprising in combination, a top lap plate and a bottom lap plate separated by the thickness of the wafers to be lapped, the top lap plate resting upon the wafers and exerting a force thereon equivalent to its weight, variable means for providing a friction force upon said wafers for causing them to slide across the inner surfaces of said plates and for causing said `friction force to decrease as said plates come closer together, means for measuring the thickness of the wafers while the machineis 1n operation, means for controlling the flow of abrasive slurry to the area between the top and bottom lap plates, and means for reducing the force of the weight of said top lap plate upon the wafers as the thickness of the wafers is reduced.

4. In a lapping machine; a top lap plate and a bottom lap plate each having a lapping surface; said top lap plate urging the lapping surfaces together with a force equivalent to the weight of Isaid top lap plate; means for opposing the weight of said top lap plate while lap.- ping is in progress; means for providing a friction force transverse to said lapping surfaces for sliding work pieces thereacross; and means for reducing said vfriction force as the lapping surfaces come closer together.

5. In a lapping machine; a top lap plate and a bottom lap plate each having a lapping surface; said lapping surface; disposed in parallel planes; said top lap plate disposed to exert its weight for providing a -force pushing the lapping surfaces together; a lifting bar attached at spaced points on said top lap plate for lifting said top lap plate while maintaining the lapping surfaces in parallel-plane relationship; cable means secured to said lifting bar; and variable means for exerting a force upon said cable means to oppose the weight of said top lap plate While lapping is in progress.

6. The apparatus of claim 5 including means for meas- 10 uring the magnitude of the force opposing the weight of said top lap plate.

References Cited in the tile of this patent UNITED STATES PATENTS 1,509,823 Blood Sept. 30, 1924 1,628,878 Gardner May 17, 1927 1,691,120 Morton Nov. 13, 1928 1,784,231 Gardner Dec. 9, 1930 1,838,553 Kadow et a1 Dec. 29, 1931 1,843,300 Player et al Feb. 2, 1932 2,033,922 Cole Mal'. 17, 1936 2,103,984 Indge Dec. 28, 1937 2,136,036 Avery Nov. 8, 1938 2,209,711 Young July 301, 1940 2,291,123 Wallace July 28, 1942 2,398,906 McCain Apr. 23, 1946 2,410,752 Sells et al Nov. 25, 1946 2,639,562 Balsiger May 26, 1953 2,688,215 Roshong et al. Sept. 7, 1954 

